1. Field of the Invention
The present invention relates to propulsion mechanisms for vessels driven by propellers, and, more particularly, to transmissions for aircraft and airboats, and to lubrication systems therefor.
2. Description of Related Art
Transmissions for driving a pair of counter-rotating propellers are known in the art for both airplanes and airboats. Such transmissions are known to be driven by belts and by gears, with input typically provided by a shaft driven by an aircraft engine operating at approximately 2500-3000 revolutions per minute (rpm). Aircraft engines are extremely expensive, noisy, and fuel inefficient.
Airboat systems that utilize belt-driven transmissions are inefficient owing to power losses caused by belt friction, especially at higher rotational velocities. Belt breakage in these systems is a source of failure. Another disadvantage of belt-driven systems is their inability to permit reduction of engine speed, since the shaft used to effect such a reduction would have to be too small to be practicable. Thus it has been deemed advantageous to utilize a different transmission method in an airboat to enable engine speed reduction without loss of efficiency.
A belt-driven, two-engine counter-rotating system has been disclosed by Stewart (U.S. Pat. No. 4,426,049). It has been taught by Becker et al. (U.S. Pat. No. 4,932,280) to use coaxial drive shaft systems for driving multiple outputs from a single input in an aircraft. Gearing means are disclosed for driving two outputs at different speeds.
A double-sprocket and wheel transmission for driving multiple propellers in the same direction is described by Fay (U.S. Pat. No. 1,329,387).
The use of a gear-based transmission for airboats has been taught by Kaye (U.S. Pat. No. 5,807,149), including a transmission for driving a pair of counter-rotating coaxial shafts, to each of which is affixed a propeller. Such an arrangement can be used with an automobile engine, which is far more economical than an aircraft engine. This transmission has been shown to reduce noise and torque, to permit varying gear ratios, to increase fuel efficiency and engine life, and to be less expensive to operate.
Improved gear-based transmissions for airboats have also been disclosed by Jordan (U.S. Pat. Nos. 5,724,867 and 6,xxx,xxx, the entire contents of both of which are incorporated herein by reference). In the ""xxx patent a lubrication system is also taught that includes a gear for driving lubricant from a well to the interior of the inner output shaft, out of that space to a pair of stiffener bearings, and into the space between the output shafts.
Another source of failure in transmissions is failure of the oil pump that heretofore has been considered an essential element.
It is an object of the present invention to provide a transmission that has improved strength and stability characteristics for driving a pair of counter-rotating propellers.
It is a further object to provide such a transmission having a single input shaft for driving means for driving the two output driven shafts.
It is another object to provide a transmission for driving coaxial counter-rotating propellers that is drivable at variable or equal speeds as desired.
It is an additional object to provide such a transmission with which it is possible to drive the counter-rotating propellers at different speeds to provide additional thrust, reduce noise output, and improve fuel efficiency.
It is also an object to provide such a transmission that is considerably lighter than previously known devices.
A further object is to provide a system and method for lubricating the output driven shafts that reduces weight and complexity.
Another object is to provide a system and method for achieving a simplified lubrication of a transmission and propulsion for an airboat.
An additional object is to provide such a system and method that eliminates the need for an oil pump to circulate lubricant.
These and other objects are achieved by the transmission of the present invention, which is for driving a pair of coaxial, counter-rotating propellers for, for example, an airboat or an aircraft, although these are not intended as limiting usages. The transmission is housed in a case that has an interior space.
A drive shaft extends from the outside of the case into the interior space and is rotatable in a first direction. When the transmission is in use on an aircraft or airboat, the drive shaft is mated at one end to a motor crank extending from and rotated by an engine. As mentioned above, previously known aircraft and airboats have utilized aircraft-type engines; however, with the transmission of the present invention, it has been found that an automobile engine can be used. This has a benefit of reducing cost, as well as other benefits to be discussed below.
A first driven shaft also extends into the interior space of the case, typically from a side opposite that of the drive shaft. The first driven shaft is for rotating an outer propeller, that is, the propeller farther from the airboat body.
A second driven shaft, which is hollow, likewise extends into the interior space of the case and is further positioned in surrounding, generally coaxial arrangement to the first driven shaft. The second driven shaft is shorter than the first, and both ends protrude beyond the ends of the first driven shaft. This second driven shaft is for rotating an inner propeller, that is, the propeller closer to the airboat body.
A gear train for driving the first shaft is housed in the interior space of the case. In its simplest configuration, the gear train comprises two gears: a drive gear and a driven gear. The drive gear is coaxially affixed to the drive shaft. The driven gear is coaxially affixed to the first shaft in such a position and configured so as to be rotatable by the drive gear. Thus, when the drive shaft rotates in the first direction, the drive gear is rotated in the first direction. This causes the first driven gear to be rotated in a second direction opposite in sense to the first direction, which consequently drives the first shaft in the second direction.
In an alternate embodiment, additional intermediate driven gears may be interposed between the drive gear and the first driven gear, so long as the total number of intermediate gears is an even number.
A sprocket train is also housed in the interior space of the case. This sprocket train includes a first and a second sprocket and a chain. The first sprocket is coaxially affixed to the drive shaft. The second sprocket is coaxially affixed to the first shaft. The chain is in engagement with both sprockets and is in such a position and configured so as to rotate the second sprocket upon the first sprocket turning. Thus, when the drive shaft rotates in the first direction, the first sprocket is rotated in the first direction. This causes the second sprocket also to be rotated in the first direction, which consequently drives the first shaft in the first direction.
Thus it can be seen that the rotation of the drive shaft in one direction achieves, through the action of the hybrid transmission comprising the gear train and the sprocket train, a counter-rotation of the two coaxial driven shafts and thus imparts counter-rotation to propellers attached thereto. There is no known system that uses fewer components than that of the present invention, which permits lower weight, improved efficiency, and enhanced reliability.
Using the present hybrid gear/sprocket transmission permits driving an automobile engine at the point of maximum horsepower, which generally implies a motor crank rotational speed approximately in the range of 5000-5200 rpm, and then gearing down the rotational speed to roughly 1200-2800, possibly even lower, a quieter speed at which to run the propellers.
The invention is not, of course, limited to the use of an automobile engine; in fact, the presence of the gear and sprocket trains enables the user to optimize for efficiency and noise characteristics by altering gear ratios as desired. An aircraft engine may also be used.
Yet another feature of the present invention is an improved lubrication system, in which an oil pump is no longer necessary, owing to the presence of the two coaxial driven shafts, between which lubricant may pass and be moved by the counter-rotation thereof. This feature of the system comprises means for injecting a lubricant into a space between the driven shafts and means for blocking the entry hole during use to retain the lubricant therein.
Among the benefits of the present invention are a minimization of components, which is believed to increase reliability and dependability, and a decrease in the weight, which increases performance and fuel efficiency. Present counter-rotator transmissions known in the art have a weight of 140-200 lbs, whereas the hybrid transmission of the present invention has a weight in the range of 90-110 lbs.